Drug delivery devices, such as autoinjectors and hand-held injectors, are commonly prescribed for patients to self-administer medication. Such devices typically include a drive mechanism (e.g., a spring) that operates on a prefilled syringe in response to a triggering event, such as the patient pressing a button on the device. The drive mechanism creates a drive force and, additionally, operates on a plunger to deliver the medication subcutaneously via the needle. These drug delivery devices may be constructed as single-use or reusable devices. Autoinjectors offer several benefits in drug delivery over conventional syringes, such as simplicity of use. Autoinjectors are beneficial for delivering drugs with high viscosities. However, as viscosity increases, the drive force required to inject the drug also increases. A large drive force may cause internal pressure build-up within the device, causing the prefilled syringe to fracture during injection.
FIG. 1 illustrates a known autoinjector 10 that includes a reservoir 12 configured to contain and/or containing a drug 11, a drug delivery member 14 configured to deliver the drug, a plunger rod 16 configured to drive a plunger 18, and a drive mechanism 20 configured to power drug delivery. The reservoir 12 in this example is a glass syringe and includes a thin-walled glass barrel 13.
The drive mechanism 20 includes a compressed coil spring 21 coupled to the plunger rod 16 and is configured to deliver an initial force to move the plunger rod 16 from a preloaded position where the plunger rod 16 is spaced away from the plunger 18, to a second position where the plunger rod 16 makes contact with the plunger 18. Upon actuation of the drive mechanism 20, this conventional autoinjector 10 can experience an impact event (not shown), where the drive force initially causes the plunger rod 16 to impart an impact force on the plunger 18 before causing the plunger 18 to move through the reservoir 12. As the plunger 18 moves through the reservoir 12, a stopper 15 of the plunger 18 is configured to sealingly and slidably engage an inner wall of the glass barrel 13 to push the drug 11 through the reservoir 12 and out through an open end of the drug delivery member 14.
Based on the requirements of the drug 11 and the force generated by the drive mechanism 20 (i.e., a high viscosity drug may require a higher drive force to move the plunger 18 through the reservoir 12), the plunger rod 16 may indirectly or directly impart impact forces onto the barrel 13 of the reservoir 12 when the plunger rod 16 impacts the plunger 18. Large forces could break the barrel 13. If the plunger 18 is placed lower in the reservoir 12, the impact becomes more likely to cause breakage. A load from the impact event generates pressure waves in the drug 11 that propagate through the glass barrel 13. For the combination of materials and geometries typical of glass syringes, a pressure wave will “couple” to the glass barrel 13 of the reservoir 12 as it propagates axially. The coupled wave oscillates through the barrel 13 and may cause the barrel 13 to fracture.